New Approach toward Laser-Assisted Modification of Biocompatible Polymers Relevant to Neural Interfacing Technologies

Abstract

We report on a new approach toward a laser-assisted modification of biocompatible polydimethylsiloxane (PDMS) elastomers relevant to the fabrication of stretchable multielectrode arrays (MEAs) devices for neural interfacing technologies. These applications require high-density electrode packaging to provide a high-resolution integrating system for neural stimulation and/or recording. Medical grade PDMS elastomers are highly flexible with low Young's modulus < 1 MPa, which are similar to soft tissue (nerve, brain, muscles) among the other known biopolymers, and can easily adjust to the soft tissue curvatures. This property ensures tight contact between the electrodes and tissue and promotes intensive development of PDMS-based MEAs interfacing devices in the basic neuroscience, neural prosthetics, and hybrid bionic systems, connecting the human nervous system with electronic or robotic prostheses for restoring and treating neurological diseases. By using the UV harmonics 266 and 355 nm of Nd:YAG laser medical grade PDMS elastomer is modified by ns-laser ablation in water. A new approach of processing is proposed to (i) activate the surface and to obtain tracks with (ii) symmetric U-shaped profiles and (iii) homogeneous microstructure This technology provides miniaturization of the device and successful functionalization by electroless metallization of the tracks with platinum (Pt) without preliminary sensitization by tin (Sn) and chemical activation by palladium (Pd). As a result, platinum black layers with a cauliflower-like structure with low values of sheet resistance between 1 and 8 Ω/sq are obtained.

Keywords: PDMS polymer; electroless metallization; implants; neural interfacing technologies; ns-laser ablation in water; platinum Pt black.

Figure 1

Experimental set-up of laser ablation of PDMS in water: (1)—Nd:YAG laser; (2)—mirror; (3)—attenuator; (4)—shutter; (5)—diaphragm; (6)—lens; (7)—PDMS sample; (8)—vessel with water; (9)—water pump; (10)—coolant tank; (11)—x-y translation stage.

Figure 2

SEM images of tracks with worsening morphologic quality obtained on PDMS surface by laser ablation in water with wavelengths of: (a) 266 nm and 28.07 J·cm⁻² and (b) 355 nm and 30.83 J·cm⁻² when bubbles are induced in the zone of the laser beam spot. The tracks are metalized with Pt.

Figure 3

HRTEM (a) and SAED (b) images of the plasma plume products deposited on copper grids during the ns-laser ablation of PDMS in air with 266 nm and fluence ~4 J·cm⁻² reveal presence of polycrystalline SiC and crystal Si phases.

Figure 4

(a) Laser track after ns-laser ablation of PDMS sample in air with a wavelength of 266 nm, the beam spot is in the focal plane, the red arrows show the debris beside the track; (b) Pt (platinum) coating deposited by electroless metallization of a track obtained by ns-laser ablation of PDMS sample in air with 266 nm wavelength, the beam spot is out of the focal plane, the red arrows show the debris beside the track, on which enough quantity of Pt is deposited.

Figure 5

SEM images in different magnifications are applied in order to demonstrate production of free of debris laser tracks after ns-laser ablation of PDMS polymer in water. No evidence of deposition of ejected material and heat-affected zone is observed near the place of ablation. The insets present higher magnification of the SEM images and images taken by optical microscope: (a) 266 nm and fluences higher than 28.07 J·cm⁻²; (b) 355 nm and fluences higher than 21.75 J·cm⁻².

Figure 6

SEM cross section images of the laser tracks produced after ns-laser ablation of PDMS polymer in water present the formation of Profiles with regular contours, and symmetric and wide-opened U-shape, with: (a) 266 nm and fluences higher than 28.07 J·cm⁻²; (b) 355 nm and fluences higher than 21.75 J·cm⁻².

Figure 7

3D Laser microscope views of the tracks produced by: (a) 266 nm laser pulses with fluence 42.66 J·cm⁻²; and (b) 355 nm laser pulses with fluence 37.68 J·cm⁻². It can be seen no debris are re-deposited after laser ablation near the tracks. HAZ also is not formed.

Figure 8

Curves fitting of XPS spectra of O1 s (a,c) and Si 2p (b,d). Laser ablation of PDMS with 266 nm: (a,b) in air with fluence 4.3 J·cm⁻²; (c,d) in water with fluence 28.07 J·cm⁻². Dashed line is the fitting curve.

Figure 8

Curves fitting of XPS spectra of O1 s (a,c) and Si 2p (b,d). Laser ablation of PDMS with 266 nm: (a,b) in air with fluence 4.3 J·cm⁻²; (c,d) in water with fluence 28.07 J·cm⁻². Dashed line is the fitting curve.

Figure 9

Raman spectra of PDMS polymer processed by 266 nm laser pulses in a water environment. Similar Raman spectra are obtained after processing with 366 nm. The Raman spectra demonstrate chemical activation of the surface after UV laser treatment with all fluences applied.

Figure 10

SEM images of electroless deposition of Pt layer on the track obtained by ns-laser ablation of PDMS (wavelength of 266 nm, fluence 35.37 J·cm⁻²) in water. High-quality metal coating only on the laser-treated area is obtained. Different magnifications are applied to demonstrate (a,b) the absence of debris near beside the laser tracks, and (c) the cauliflower-like structure of the Pt layer, consisting fine nanograins.

Figure 10

SEM images of electroless deposition of Pt layer on the track obtained by ns-laser ablation of PDMS (wavelength of 266 nm, fluence 35.37 J·cm⁻²) in water. High-quality metal coating only on the laser-treated area is obtained. Different magnifications are applied to demonstrate (a,b) the absence of debris near beside the laser tracks, and (c) the cauliflower-like structure of the Pt layer, consisting fine nanograins.

Figure 11

EDX image presented an elemental spectrum of electroless deposited Pt layer on the track obtained by ns-laser ablation of PDMS (wavelength of 266 nm, fluence 35.37 J·cm⁻²) in water, confirming that the metal crystals are truly platinum.